Tuning of a conventional Distributed Bragg Reflector (DBR) semiconductor laser is limited by the fact that the relative tuning range is restricted to the relative change in the refractive index of the tuning region. This means that the tuning range, under normal operating conditions, cannot exceed 10 nm. This is substantially less than the potential bandwidth, restricted by the width of the gain curve, which is about 100 nm. Such conventional DBR lasers can functionally be characterized as comprising a first part being a two-sided active section, for creating radiation, for instance a light beam, by spontaneous emission over a bandwidth around one center frequency. Said first part also guides said radiation or light beam. Such conventional DBR lasers further have two reflectors. Said reflectors are bounding said two-sided active section, thus one at each side.
The limited selectivity problem has been recognized by Wolf, et al (European Transactions on Telecommunications and Related Technologies, 4 (1993), No. 6) showing in FIG. 10 a laser structure with two parallel waveguides but without gratings. These two parallel waveguides cannot be considered as resonators, indeed the spectra (shown in FIG. 10b and c) show the comb mode spectra corresponding to arms B and A, but they are either the comb mode spectra of arm B, the gain section and the reflectors R or the comb mode spectra of arm A, the gain section and the reflectors R. As the spacing between the spectral lines are determined by the length of the structures, it appears that said spacing is still very small, resulting in still a low selectivity and a low tuneability.
Over the past years several advanced laser structures have been proposed with an extended tuning range. Examples are the Y-laser [M. Kuznetsov, P. Verlangieri, A. G. Dentai, C. H. Joyner, and C. A. Burrus, “Design of widely tunable semiconductor three-branch lasers,” J. Lightwave Technol., vol. 12, no. 12, pp. 2100–2106, 1994], the co-directionally coupled twin-guide laser [M.-C. Amann, and S. Illek, “Tunable laser diodes utilizing transverse tuning scheme,” J. Lightwave Technol., vol. 11, no. 7, pp. 1168–1182, 1993], the Sampled Grating (SG) DBR laser [V. Jayaraman, Z. M. Chuang, and L. A. Coldren, “Theory, design and performance of extended tuning range semiconductor lasers with sampled gratings,” IEEE J. Quantum Electron., vol. 29, no. 6, pp. 1824–1834, 1993], the Super Structure Grating (SSG) DBR laser [H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuous wavelength tuning in super-structure-grating (SSG) DBR lasers,” IEEE J. Quantum Electron., vol. 32, no. 3, pp. 433–440, 1996] and the Grating assisted Coupler with rear Sampled Reflector (GCSR) laser [M. Öberg, S. Nilsson, K. Streubel, L. Bäckbom, and T. Klinga, “74 nm wavelength tuning range of an InGaAsP/InP vertical grating assisted codirectional coupler laser with rear sampled grating reflector,” IEEE Photon. Technol. Lett., vol. 5, no. 7, pp. 735–738, 1993]. In the first two types of devices, a trade-off had to be made between the tuning range and the spectral purity (broad tuning range vs. high Side Mode Suppression Ratio (SMSR)). Therefore recently most research attention has gone to the (S)SG-DBR and GCSR lasers.
A sampled grating DBR laser, comprises of two sampled gratings exhibiting a comb-shaped reflectance spectrum, with slightly different peak spacing due to the different sampling periods. As an alternative, other grating shapes can be used: these are normally referred to as “super structure gratings” (SSG). Lasers of this type have been fabricated with tuning ranges up to about 100 nm. The operation of the device is such that through current injection in the two DBR sections, a peak of the front and rear reflectance comb are aligned at the desired wavelength. The phase section is used to align a longitudinal cavity mode with the peaks of the two reflectors. The disadvantage of the (S)SG-DBR approach is that light coupled out of the laser has to pass a long passive or inactive section, leading to loss. Also, the losses in the two reflector sections increase with the amount of current injected into those sections, leading to a tuning current dependent output power.
The SG-DBR laser and the SSG-DBR laser are functionally characterized as comprising a two-sided active region for light creation and two reflectors one at each side of the active region, said reflectors having a reflection characteristic with a plurality of reflection peaks. Said characteristic has spaced reflection maxima points providing a maximum reflection of an associated wavelength. Such a characteristic can be obtained via sampled gratings, which exhibit a comb-shaped reflection spectrum or via the so-called supergratings. Said gratings or supergratings can also be characterized as distributed reflectors.
Sampled gratings can be described as structures in a waveguide system, having a periodically broken short-period structure including short period stripped regions alternating with non-stripped regions. The supergratings can be described as structures in a waveguide system having a diffractive grating having a plurality of repeating unit regions, each having a constant length, thus forming a modulation period, and at least one parameter that determines the optical reflectivity of said diffractive grating varying depending on its position in each of said repeating unit regions along a direction of optical transmission in said laser, said diffractive grating extending by at least two modulation periods. Reference is made to U.S. Pat. No. 5,325,392 related to distributed reflector and wavelength tunable semiconductor lasers, which is hereby incorporated by reference in its entirety.
The SG-DBR laser and the SSG-DBR laser exploit constructive interference of the periodic characteristics of reflectors, located at different sides of the active section, with different periodicity, to obtain a wide tunability. The alignment of the reflector peaks can be described by stating that the spacing of said reflective maxima points of the reflectors are different or are essentially not equal and only one said reflective maxima of each of said reflectors is in correspondence with a wavelength of said created lightbeam. Reference is made to patent U.S. Pat. No. 4,896,325, related to multi-section tunable lasers with differing multi-element mirrors, which is hereby incorporated by reference in its entirety.
As the construction of said reflectors leads to long inactive sections, this results in lasing output power losses.
Other lasers, which use a co-directional coupler, readily have a very wide tuning range, but there is insufficient suppression of neighbouring longitudinal modes. The combination of a widely tuneable but poorly selective co-directional coupler with a single (S)SG reflector will give both wide tuning and a good side mode suppression. Furthermore, the optical output signal does not pass through a passive region. Again tuning of 100 nm has been achieved. Unfortunately, such a structure is rather complicated to manufacture, requiring at least 5 growth steps. Reference is made to patent U.S. Pat. No. 5,621,828 related to integrated tunable filters, which is hereby incorporated by reference in its entirety.
EP-A-0926787 describes a series of strongly complex coupled DFB lasers. In the disclosed structure, gratings are made within the active sections. Said gratings are selected such that no substantial interaction between the lasers, defined by a grated active section, in series is obtained. The disclosed structure enables generation of multiple wavelengths, even sumultaneously, but does not address the issue of selectivity and tuneability.
A parallel structure with a plurality of waveguides is disclosed in the PATENT ABSTRACT OF JAPAN, vol. 013, no. 026, 20 Jan. 1989, JP 63 229796 (Fujitsu Ltd. The disclosed structure again enables radiation of a plurality of wavelengths but does not address the issue of tuneability. The optical switch is operated for selecting a waveguide, thus no simultaneously optical connection between said waveguide is obtained.